The 18-year-old, who has made it past the third round of a Grand Slam for the first time, next faces 10th-seeded Czech Karolina Pliskova, a 4-6, 6-4, 7-6 (7/3) winner over Venus Williams.

Ana Konjuh shakes hands with Agnieszka Radwanska. Reuters

The victory over Radwanska was sweet revenge for Konjuh, who held three match points against the Pole in the second round at Wimbledon but eventually lost 9-7 in the third set after stepping on a ball and turning her ankle.

It was another blow for a player who starred as a junior, winning Australian Open and US Open junior titles in 2013, before she was slowed by elbow surgery in 2014.

"I'm still young. I still need to experience things," she said. "I think I didn't have that many matches as I should. It was tough to come back every time. But, you know, here I am in the quarter-finals of the Grand Slam, so I cannot complain."

Regardless of any lack of experience Konjuh showed plenty of poise on the massive Arthur Ashe stadium court, even when the sound system blared out suddenly as she prepared to serve early in the final game.

She kept her nerve, giving herself a match point with a blistering backhand down the line, and claimed the win with a serve that Radwanska could only smack into the net.

"This time I took opportunities," she said. "I was just going for it. It was the key of the match."

For Radwanska it was more US Open heartbreak. It remains the only Grand Slam where she has never reached the quarter-finals.

"Well, she was playing pretty good tennis, serving very well," said Radwanska, who broke Konjuh in the second game but immediately gave the break back.

Konjuh, who finished with six aces and 38 winners, broke again for a 4-3 lead and never let go.

"I just couldn't do any more I guess today," Radwanska said. "I was really trying. I was just too slow today."

In a tight second set, neither player faced a break point until Konjuh raced ahead to 0-40 in the ninth game, giving herself a chance to serve it out with a backhand volley winner.

"Definitely it was only one break in the beginning, then I was not really close to breaking again," Radwanska said. "I didn't serve good enough. I'm not here as a winner."